The present invention relates to a method for forming a porous covering layer on a substrate.
Hitherto, a covering layer has been formed on various substrates for various reasons.
For example, a developing roller of, e.g., a copying machine or a laser printer has a polyurethane covering layer disposed on a conductive rubber layer formed on a metal core to prevent the surface of a photosensitive drum from being polluted by materials transferred from the conductive rubber layer.
Further a non-reflective covering layer is disposed on the inner surface of a camera or a lens-barrel.
However, conventional covering layers cannot keep their initial performance as they are worn away. Thus, they are unsatisfactory in that respect.
Therefore, a main object of the present invention is to provide a method for forming, on a substrate, a covering layer which can substantially keep its initial performance even if it is worn away.
The inventors have conducted researches in an attempt to attain the above-mentioned object. As a result, the inventors have found that the object can be attained by reacting, on a surface of a substrate, a polyol with an isocyanate compound and optionally reactive silicone oil in the presence of a volatile silicone oil to produce a polyurethane covering layer on the substrate. The volatile silicone oil is volatilized during the reaction, and thus acts as a pore-forming agent to make the polyurethane covering layer porous.
Accordingly, the present invention provides a method for forming a porous covering layer on a substrate, comprising applying a reaction mixture containing a polyol, an isocyanate compound, and a pore-forming agent comprising a volatile silicone oil onto a surface of the substrate, and subjecting the reaction mixture to conditions under which the polyol reacts with the isocyanate compound.
The present invention also provides a method for forming a porous covering layer on a substrate, comprising applying a reaction mixture containing a polyol, a reactive silicone oil having active hydrogen, an isocyanate compound, and a pore-forming agent comprising a volatile silicone oil onto a surface of the substrate, and subjecting the reaction mixture to conditions under which the polyol and reactive silicone oil react with the isocyanate compound.
The polyol in the present invention is preferably a fluorine-containing polyol, and more preferably a copolymer polyol comprising a tetrafluoroethylene monomer unit as a major component.
The reactive silicon oil is preferably represented by the following formula (1) or (2): 
where each R represents xe2x80x94C3H6OC2H4OH or xe2x80x94C3H6OCH2xe2x80x94C(CH2OH)2C2H5 and n represents an integer of not more than about 20.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.